Samples for indentation experiments

Samples for indentation experiments were cold sintered prior to the testing. To this end, 0.3 g of particles were mixed with 0.075 ml of DI water, poured into a pressing tool (Modell 10, Gr. 2, diameter 11 mm, P/O Weber, Germany) and pressed at 500 MPa for 1 hour using a mechanical testing machine (Instron 8562, Instron). The resulting pellets were 87 % dense. The pellets were fixed onto the stage for indentation using a wax. The indentation was performed in a NanoTest Vantage from Micro Mechanics (Wrexham, U.K.) using a Berkovich tip. For the microindents, the force was increased with a speed of 80 mN/s until a holding force of 4800 mN was reached. The sample was then unloaded with the same speed. For the nanoindents, we programmed the instrument to perform an array of 100 indents (10 by 10). In this case, the force was increased with a speed of 0.25 mN/s until the holding force of 5 mN was reached. The sample was unloaded with the same speed. Specimens measured under wet conditions were soaked in water prior to the test. A wet sponge was attached next to these samples to minimize water evaporation during the measurement.